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Inability to taste and respond to fat in food may explain why some overeat: Study

You might think twice before taking a bite out of a Big Mac once Ontario requires all chain restaurants to post calorie counts right on their menu boards. (Joe Raedle/ Getty Images) Photo: Joe Raedle/Getty Images/File

New research is offering a tantalizing clue as to why some people overeat: “Impaired oral fatty acid chemoreception,” or the inability to taste and respond to the fat in foods.

People whose mouths and gastrointestinal tracts are less sensitive to fatty acids — the breakdown products of fat — require higher concentrations of fat to generate signals from the gut that tell the brain, “I’m full.”

No fullness signal, and people keep eating, explains Russell Keast, of Deakin University in Australia. “The overconsumption eventually leads to obesity.”

In a new experiment appearing in the journal, Appetite, Keast and colleagues report that people with diminished fat sensitivity ate significantly more calories and more food at lunch after having a high-fat breakfast than those whose fat receptors are fully functioning.

The study builds on earlier research by Keast suggesting fat is the sixth sense of taste, behind sweet, salty, sour, bitter and the more recently added umami, the flavour associated with monosodium glutamate, or MSG.

Humans have receptors on their tongues for the breakdown products of carbohydrates (sugars, which give foods their sweet taste), as well as protein (amino acids, in particular glutamate). “So, it makes logical sense we would have receptors for the other macronutrient, fat,” Keast, a professor in the Centre for Physical Activity and Nutrition Research at Deakin University in Victoria, Australia, said in an email.

In the new study, 24 volunteers were tested for fatty acid sensitivity. Each then ate four different breakfasts — a high-fat breakfast (variations of frittata), high-carb breakfast, high-protein or a nutritionally balanced meal — over four separate days.

Photo: Viktor/Fotolia.com

Participants were told not to consume any food or beverages (except for water up to an hour before lunch) between breakfast and lunch. For lunch, a buffet-style meal was served that included soup, baked pasta, and salad, grapes, pastries and chocolate bars. People were instructed to eat until comfortably full.

Following the high-fat breakfasts, the 10 people with impaired fat sensitivity consumed significantly more calories and grams of food at lunch compared to the other volunteers. But there were no significant differences in the amount of food consumed at lunch after the other breakfasts.

It’s as if the fat is somehow “invisible” as it passes through their mouths and into their stomachs, Keast explained. “Subjects who were insensitive did not receive the fullness signals from the fat like the sensitive subjects did. Therefore, they consumed significantly more energy.”

They felt hungrier after the high-fat breakfasts, but not the high-carb or high-protein breakfasts. That suggests the effect was specific for fat.

Keast is looking at why some people are more susceptible than others and whether it’s possible to increase a person’s fat sensitivity.

The proportion of the population insensitive to fatty acids depends upon where the thresholds are set, he said. However, on average, 40 per cent appear to be insensitive when tested.

One possible way people might tell whether they have impaired fat receptors would be comparing how their dinner companions respond after consuming a similarly fatty meal, Keast said. “Are they full and content, and are you still hungry, wanting more to eat an our or so later?”

The study was small in size and involved mostly young, healthy weight volunteers. But in an earlier study, Keast and colleagues reported that people hypersensitive to fatty acids eat fewer calories and fat, and are less likely to be overweight or obese than people with a “hypo” or low sensitivity to fat.

A leading Canadian obesity expert said the new findings may partly explain why obesity rates have been rising despite the “low-fat craze” of recent years.

The study suggests, “not eating enough fat (at least in people with an impaired ability to taste fat) may well lead to overeating,” Dr. Arya Sharma, professor of medicine and chair in obesity research and management at the University of Alberta in Edmonton writes in his blog, www.drsharma.ca.

While more research is needed, “I wonder if we will soon be advising our patients to avoid low-fat products as a means of eating fewer calories,” he said.